1. Field of the Invention
The present invention relates to disc-type filtration apparatus employed for separation of liquid from a suspension or slurry, such as a lime mud. More specifically, the invention relates to a disc-type filtration apparatus housed within a pressure vessel and employing a positive pressure in conjunction with filter discs to effect the liquid separation, as well as a method of filtration.
2. State of the Art
Disc-type filtration apparatus are well known, and have been employed for decades. In its simplest form, such an apparatus employs one or more hollow filter discs, the sidewalls of which discs are either formed of a porous material or define a frame covered with a porous cover, commonly termed a filter cloth. The degree of porosity is selected for the suspension to be filtered, considering the viscosity of the liquid component as well as the particle size or sizes of the solid component. The discs are usually circumferentially sectored into compartments, are oriented vertically and mutually laterally spaced along a tube extending along a horizontal axis. The interiors of the disc sectors are in communication with the interior of the tube. During operation, the discs are rotated about the longitudinal axis so as to become coated with the suspension. A positive pressure differential is created between the exteriors of the discs and the interiors thereof, to draw much of the liquid component of the suspension coating the disc exteriors into the interiors and significantly reducing the liquid content of the suspension on the disc exteriors to form a “cake.” The cake is then removed from the disc exteriors.
Two principal approaches have been used to create the pressure differential between the disc interiors and exteriors. In one approach, exemplified by the teachings of U.S. Pat. Nos. 2,699,872 and 2,899,066, a vacuum is drawn in the tube and the exteriors of the discs are at ambient atmospheric pressure. A currently more favored approach, due to its ability to provide a greater pressure differential, particularly in the case of large, multi-disc filters, is to deploy the filter discs within a pressure vessel which is pressurized above ambient, as by an air compressor. This latter approach is exemplified by the teachings of U.S. Pat. Nos. 2,079,755; 3,252,577; 5,849,202; 6,063,294; Canadian Patent No. 1,192,142 and International Patent Application No. WO 90/10490. A commercially available filtration apparatus of the latter type is offered by EIMCO Process Equipment Co. (formerly Baker Process) of Salt Lake City, Utah, assignee of the present invention, as the ClariDisc® Filter. Another commercially available apparatus of the latter type is offered by Kvaerner Pulping AB of Karlstad, Sweden, as the CAUSTEC® PDW™ filter.
The design of the aforementioned ClariDisc® Filter has been well accepted by customers, as the solids from this filter (comprising filter cake which has been reslurried), are collected in a sloping outlet trough portion integral with and protruding beyond the cylindrical envelope of a cylindrical pressure vessel portion housing the discs, and discharged from the pressure vessel through a single outlet or multiple outlets. This is in contrast to the CAUSTEC® PDW™ filter, which uses a plurality of discharge outlets extending from the exterior of the pressure vessel, one outlet for each disc. The CAUSTEC® PDW™ filter is unduly complex and expensive to both build and deploy, due to the use of multiple outlets extending from the cylindrical pressure vessel portion. However, it is also somewhat costly to design and manufacture the trough and outlet assembly for the ClariDisc® Filter, since it functions as part of the outer wall of the pressure vessel and, therefore, must comply with certain governmental or professional engineering standards, the latter exemplified by the American Society of Mechanical Engineers (ASME) Section 8 Pressure Vessel Code.